This invention relates to a self-correcting protective circuit for establishing a reference voltage for the conductive backplane used to interconnect circuit boards, preferably for use in an office interface unit in a telephone central office or remote extension thereof.
In electronic system it has become common practice to utilize multilayer printed circuit boards separated by dielectric layers, for example vinyl sheet materials, from planar conductive layers. One such conductive layer is referred to as the backplane and contains a plurality of busses which are normally maintained in a known state and serve to distribute or supply signals applied thereto to other printed circuit boards through interconnections. In the field of telecommunications, the backplane busses are interconnected to a number of signal generators and signal receivers via physical connectors such as pin connectors. The backplane is maintained at a normal operating voltage which changes upon receipt of a signal from an outside source. The interconnections with other boards can introduce discontinuities into the circuit paths and create a need for impedance matching circuits at the backplane. The impedance matching circuits operate at voltage levels that typically are substantially different from the voltage level of the system power supply. In the telecommunications field, it is customary to utilize a power supply operating at 48 volts with a 5 volt reference voltage established on the backplane busses and used for the associated impedance matching circuits.
This lower voltage can be derived form the 48 V power supply by utilizing one or more DC-to-DC converters. Since the designed-for power requirements of the circuitry coupled to the backplane remain essentially constant, the direct lowering of the backplane voltage results in a corresponding increase in the current required for the backplane. To avoid having to supply a large current from a single source to the backplane, it has been proposed to utilize a higher reference voltage at the backplane and to provide a separate DC-DC converter on each circuit board, as illustrated in FIG. 3. The individual voltage converters then are coupled to the backplane and establish the reference voltage therefor. The plurality of individual converters are electrically connected in parallel to the backplane.
The use of a parallel configuration of circuits providing a common reference voltage at the backplane on a backplane bus suffers from the problem that a slight mismatch of any of the operating characteristics in the individual circuits results in unequal loads being shared by the circuits. For example, one of the circuit boards may have a higher voltage level than other boards in the parallel configuration. As a consequence, this circuit board will drive the backplane and other associated circuit boards thereby resulting in an unequal sharing of the power. Protection circuitry would have to be provided on each circuit board in order to prevent the unequal sharing of power from damaging the board during operation. This is a costly solution to the problem.
Alternatively, the output terminal of each voltage converter can be coupled to a backplane bus through the serial combination of a diode and a current-limiting resistor, as illustrated in FIG. 4. Each diode serves to prevent current flow back to the converter at each board while the resistor functions to reduce the load imposed on any one circuit board should it carry the higher voltage level. Typically, the resistor is only 1 ohm. Accordingly the protection it provides is only in the case of mismatches between the operating characteristics of the converters. In the event of a short circuit occurring, the current demand on each of the converters will exceed the ability of the resistor to provide any significant current limiting function. As a result, the converters are then protectively shut down, or fail, thereby resulting in the telecommunications equipment ceasing to operate.
Accordingly, the present invention is directed to a circuit for establishing a reference voltage for a backplane and other circuits while providing protection against a short circuit load condition. The circuit utilizes transistors that are maintained in the conductive state so as to be responsive to changing conditions. Also, the circuit corrects to normal operating conditions rapidly. In addition, the circuit is configured to approximate the ideal reference voltage source in that the internal impedance is maintained low in the forward direction.
The present invention has a primary objective the provision of a reference voltage source which can be used individually or in parallel to establish a reference voltage at a backplane in telecommunications equipment.